1. Field of The Invention
The invention relates to the passage of a transit line through a conduit containing straight sections and bends. More particularly, it relates to the overcoming of the capstan effect that impedes the passage of the line through such a conduit.
2. Description of the Prior Art
The Sandjet process is a well known and successful process for the in-situ cleaning of the interior surfaces of conduits used for the transport and/or processing of fluids, solids or a mixture thereof. The conduits thus cleaned include fired heater tubes used in hydrocarbon or chemical processing, pipelines, heat exchange tubes and the like. In the practice of the Sandjet process for such in-situ cleaning operations, cleaning particles are entrained in a propelling fluid stream and are introduced into the conduit to be cleaned at a velocity sufficient to effect the desired cleaning action.
In furnace tube applications, the Sandjet process is used to decoke and clean furnace tubes. This application of the process is described in the Nunciato et al patent U.S. Pat. No. 4,297,147, entitled "METHOD FOR DECOKING FIRED HEATER TUBES". As is well known to those skilled in the art and as disclosed and illustrated in said patent, furnace tubes generally comprise a series of straight sections and return bends. In some instances, the tubes will have an equivalent continuous helical tube configuration. By the use of steel shot or other suitable cleaning materials, the Sandjet process can achieve a desirable decoking action without undue abrasion of the straight sections or of the return bends of such furnace tubes. The Sandjet process provides significant advantages over the known alternative decoking approaches, such as turbining, hydroblasting and steam-air decoking, as is noted in the patent. There is also a growing appreciation in the art of the energy savings that can be derived when furnace tubes have been decoked by means of the Sandjet process as compared with the results obtainable by the most frequently used alternative approach, i.e. steam-air decoking.
It is recognized, however, that improvements in the Sandjet process are required in order to extend the capability of the process for the cleaning of certain furnace tubes having difficult-to-remove coke deposits on the interior surfaces thereof. Such deposits, as present in certain high heat duty or thermally abused furnace applications, are not removed to the extent desired by the Sandjet process using steel shot as the cleaning material. lt has also been found that such deposits are also generally resistant even to angular, abrasive cleaning materials, such as flint. The use of such abrasive materials are not generally desirable for use in furnace tube decoking applications in any event, as such materials cause severe erosion of the bends of furnace tubes, even in those instances in which difficult-to-remove deposits may not be satisfactorily removed from the straight sections of the tubes. The improvements needed in the art should enable the Sandjet process to be employed with enhanced reliability in the decoking of difficult-to-remove deposits, out without resulting in an unacceptable level of abrasion of the tubes, particularly the bends of said tubes.
One approach to the development of improvements enhancing the Sandjet process resides in the use of new cleaning agents to achieve an advantageous balance of desired cleaning action and undesired aorasive action. Some such agents would have an improved cleaning action over that achieved by steel shot, while avoiding tne abrasive action of materials such as flint. Other cleaning agents might also have an enhanced cleaning effectiveness, particularly with respect to difficult-to-remove deposits, where such effectiveness is of primary concern and where the furnace tube system can tolerate the higher level of erosion that may accompany the ability of the agents to effectively remove deposits that are resistant to removal when other, less abrasive cleaning materials are employed.
It may, nevertheless, oe desirable to use steel shot or relatively mild abrasive materials in particular applications of the Sandjet process for furnace tube decoking or other conduit cleaning operations. It will also be appreciated oy those skilled in the art that, for certain decoking or other cleaning operations, further improvement in the Sandjet process may be desirable over and above that achievable by enhanced cleaning agents alone. In such instances, other techniques for enhancing the effectiveness of the Sandjet process are desired to meet the requirements of particular commercial applications. The use of a flow diverter to divert the cleaning particles entrained in a propelling gas stream toward the inside surfaces of the furnace tube or other conduit to be cleaned has been considered for such a purpose.
The use of a flow diverter to improve the cleaning of the inside of a pipe is known in the art. The Prince patent, U.S. Pat. No. 2,745,231, for example, teaches the insertion into a pipe of a generally spherical, rigid object having a diameter slightly less than the inside diameter of the clean pipe. The object almost blocks the bore, but is nevertheless free to pass through it upon being subjected to hydraulic pressure. When the object, such as a solid or hollow steel ball, is in the pipe and an abrasive-laden liquid is being forced through the pipe, the pumping pressure applied to the abrasive-laden liquid, and the action of the object in directing the liquid into the annular space between the object and the surface of the pipe, create an abrasive jet against the pipe to remove scale deposited thereon. Similarly, the Messer patent, U.S. Pat. No. 335,608, shows the use of a ball placed inside a pipe and tethered by means of a cord, to force a stream of water to flow around its periphery to strike and dislodge sediment deposited on the surface of the pipe. The Fritze patent, U.S. Pat. No. 2,739,424, shows the use of a conically shaped deflector to deflect abrasive material against the inner surfaces of a pipe being sandblasted. The use of such a flow diverter in the practice of the Sandjet process, for furnace tube decoking or other cleaning applications, would be beneficial to the in-situ cleaning action being carried out within the tube or other conduit. Thus, a flow diverter, such as a cone or sphere, placed in a propelling gas stream having cleaning particles entrained therein, would divert the particles toward the pipe or conduit wall, increasing the angle of impact and the number of impacts of the cleaning particles with the inside wall surfaces. The velocity of the particles and of the propelling gas stream would also be increased in the vicinity of the flow diverter. The use of a flow diverter in the practice of the Sandjet process would thus be advantageous in certain applications, enhancing the effectiveness of the cleaning action and perhaps enabling steel shot or mild abrasive cleaning agents to approach the cleaning effectiveness of flint, grit or other abrasive materials, but without the relatively severe bend erosion commonly associated with such more agressive, abrasive cleaning materials.
In the decoking of furnace tubes having many straight sections and return bends, it has not heretofore been possible to employ a flow diverter with sufficient success to justify its incorporation in the Sandjet process on a practical commercial basis. In the absence of a tether or other restraining or pulling means, the free-flowing diverter may become hung-up or jammed in the tubes being cleaned in a position such as to make it difficult to dislodge and remove the diverter from the tubes. In such a circumstance, costly and time consuming efforts may be required in order to remove the diverter from the tubes, even to the extent of requiring that the tubes be cut open in order to recover the flow diverter and open up the interior surface of the tubes for further cleaning before re-use. If the diverter does not become jammed, it is found to pass through the tubes at such rapid, uncontrollable speed that it is ineffective for its intended cleaning enhancement purposes. The use of a tether or connecting line is essential, therefore, to avoid such undesirable circumstances that may render the use of the diverter futile or even totally destroy the benefits to be achieved by the use of the in-situ Sandjet process in the first instance. It has been found, however, that a tethered flow diverter cannot be used in the practice of the Sandjet process for the decoking and cleaning of furnace tubes having a series of five or more straight sections and return bends as occurs in typical furnace tube bundles. As the line or other tether is wrapped about return bend sections, and extends along straight sections, and back around the next succeeding return bend and continues in the opposite direction in the next succeeding straight section, throughout a series of many straight sections and return bends, a capstan effect is encountered that effectively precludes the passage of the diverter through the tubes. This effect is due to the very appreciable frictional forces encountered in attempting to move a line through a flow path having such a back and forth movement through a series of many straight sections and return bends. It will be appreciated that such forces pertain regardless of whether the line and the flow diverter attached thereto are being pulled against a flow of cleaning material and a propelling gas stream, or are attempted to be moved by such gas stream against the restraining frictional force of a tether. In either case, it has been found that the frictional forces encountered due to the capstan effect are sufficiently great as to effectively preclude the use of a tethered flow diverter in practical commercial operations. Thus, the very great forces involved may exceed the strength of the line or the capacity of the propulsion means employed to move the line through the tubes, or require the use of pressures exceeding the strength of the tubes themselves, or may otherwise render the use of a tethered diverter impractical in Sandjet process operations or in other applications in which it is desired to pass a tether, cable or other transit line through the interior of a conduit having a series of straight sections and bends. The overcoming of the capstan effect would be useful, therefore, not only to enable a flow diverter to be used in the practice of the Sandjet process for furnace tube decoking, but more broadly to enable any transit line to be passed through a series of straight sections and bends without the overwhelming deterrent created by the capstan effect, regardless of the particular purpose for which the line may be advantageously passed in innumerable practical, commercial activities.
It is an object of the invention, therefore, to provide a process for overcoming the capstan effect encountered in passing a line through such a conduit, thus facilitating the passage of said line through a conduit having a series of straight sections and bends.
It is another object of the invention to enhance the Sandjet process for the in-situ cleaning of conduits having such straight sections and bends.
It is a further object of the invention to provide a process in which the beneficial effects of a flow diverter can be utilized in the practice of the Sandjet process for the decoking and cleaning of furnace tubes.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.